On Jul 7, 8:42*pm, Kenneth Scharf wrote:

A pale blue glow can be a normal ionization glow, doesn't mean gas.
The tank circuit on most rigs for 80 meters tends to be low Q which
means it will tune broadly. *If the the plate current dip and max power
output doesn't match it could mean a miss-match between your antenna and
the rig, IE: high SWR. *The tank circuit on the '401 is designed to
match between a 50 ohm load, stray too far from that and the tank
doesn't tune correctly, 80 meters is the worst in this regard due to the
limited range of the variable caps on that band.

No grid current? *check your bias setting, driver tube OK?- Hide quoted text -

The maximum-output-not-at-plate-current-dip problem occurs when I use
my 50 Ohm dummy load.

Incidentally, the 401 is specified for output (antenna) loads between
50 and 75 Ohms. I suspect this is why the LOAD control has little
effect on the tuning.

Honestly, it's the no-grid-current problem that has me more puzzled.
I've checked the grid meter circuit and found the resistors to be OK.
Just for kicks, I may open up the 401 and check the them again. The
meter works fine for other measurements (plate current, peak output
voltage, etc.), so I don't think it's the problem.

-Dave, K3WQ